The present invention generally relates to apparatus and methods for an emergency power system. More specifically, the present invention relates to an emergency power system comprising a plurality of solid propellant gas generators operable to produce a quantity of gas at a sufficient pressure for delivery to a turbine power unit (TPU) to provide output power as a part of an emergency power system (EPS).
Vehicles such as aircraft may include an environmental control system for providing temperature conditioned, pressurized air to the pilot, passengers, and heat generating electronics; and an auxiliary power unit may be used to supply auxiliary power for the aircraft, typically when either the aircraft is on the ground or flying at low altitudes (e.g., below about 30,000 ft). The aircraft may also include an emergency power unit to provide emergency power in the event of a failure or other condition that may threaten operation of the vehicle. An emergency power unit may include a self-contained fuel and oxidizer supply commonly known as a stored energy system, which may be insensitive to altitude and which may be operable to be activated quickly for rapid starting of the emergency power system.
Systems that may be powered by an emergency power system may include, for example, a hydraulic power supply system that may be utilized to supply hydraulic power for components and systems required to guide the aircraft, and an electrical generator that may be utilized for providing electrical power to various electrical systems within the aircraft. Both the hydraulic power supply system and the electrical system may be powered by a gas-driven turbine rotationally connected thereto. In an emergency situation, the gas which may be used to drive this turbine may be produced by the emergency power system through combustion of a propellant within a gas generator, which may be ignited upon demand.
Gas generators or combustors which may be used in an emergency power system may include a liquid propellant system, which may include a two part liquid propellant system, or a single liquid propellant system. Liquid propellant systems may have the advantage of being turned on and off as the need arises. Examples of prior art liquid propellant systems include that disclosed in U.S. Pat. No. 4,864,812, directed to an emergency power unit comprising a two part liquid propellant comprising a compressed oxidizer, and a fuel which may be combusted in a combustor to produce an amount of gas at a pressure suitable for use in generating power. However, such systems may require tanks, pumps, controls and the like to store the oxidizer and to meter delivery of the oxidizer and the fuel during operation. Accordingly, a two-part liquid propellant system may add complexity, weight, and cost to an aircraft.
U.S. Pat. No. 4,505,105 is directed to a one-part or single fuel gas generation system that utilizes hydrazine, or the like, as a fuel to generate gas in an amount and at a pressure suitable for use in powering an emergency power unit. In such a system, a catalyst bed is used to convert the hydrazine or other single propellant into a gas. While such systems may reduce the weight and cost associated with storing and delivering an oxidizer, such systems, may require storage and servicing of a highly reactive fuel, and maintenance of a catalyst bed. Also, single fuels (e.g., hydrazine) of a one part gas generation system may be toxic, and thus may present safety issues related to fuel toxicity, as well as to explosion hazards. Maintaining catalyst bed integrity, and preventing fouling or poisoning of the catalyst, may also challenge the reliability of such systems.
A solid propellant may also be utilized within a gas generation system to produce an amount of gas at a pressure suitable to drive a gas turbine of, for example, an emergency power unit. Solid propellants may be beneficial from a safety and reliability point of view, especially in single use applications. For example, U.S. Pat. No. 4,599,044 is directed to a thrust vector control system for use in a guided missile, in which a solid propellant gas generator is ignited to power a turbine-driven pump for driving hydraulic actuators for controlling systems on the guided missile. However, unlike liquid propellant systems, solid propellant systems may not be turned on and then turned off as the need arises. Once a solid propellant gas generator is actuated (i.e., begins to burn), it may be required to continue to do so until the solid propellant is exhausted.
The design of solid propellant gas generators used for an application with an unknown duty cycle, or an unknown output power requirement over a period of time, may be required to be sized to a “worse case” proportion, such that once actuated, the solid propellant gas generator may be capable of supplying enough gas to operate all the systems which could be affected, even though all of these systems may not be affected in a particular case. In addition, solid propellant may be heavier than are liquid counterparts, especially in light of the need to oversize such a unit to a worse case proportion.
Other factors that may affect the usefulness of a solid propellant gas generator may include the burn rate of the solid propellant, which may be affected by a number of variables. The composition of the solid propellant may affect the burn rate of a solid propellant. In addition, the solid propellant burn rate may be sensitive to initial propellant temperature e.g., the solid propellant may burn faster if the propellant is warm, and slower if the propellant is cold. The burn rate may also be sensitive to the gas pressure developed within a chamber of the solid propellant gas generator as the solid propellant burns e.g., high pressures in the chamber may increase the burn rate. The rate at which the solid propellant burns thus may determine the quantity and the pressure of the gas delivered to a turbine inlet of a gas turbine housing.
Also, the power demand of an aircraft may not be constant, and may thus include periods of high energy, usage which may require periods of high gas flow for power generation. An aircraft may also experience periods requiring much lower energy usage, thus requiring much lower gas flow demand. In order to prevent overspeed of the gas-driven turbine during periods of low power demand (e.g., due to an excess of gas being supplied to the turbine), an artificial load may be maintained on the power generation apparatus, e.g., a hydraulic relief valve may be included in a hydraulic circuit to maintain loading on a pump, the excess energy may then be dissipated as heat. This approach however, may require increasing the size of an oil reservoir or other components which supply oil to the hydraulic circuit, which may add weight to the aircraft. The gas generator and related propellant sizing and weight may also have to be increased for this approach.
Solid propellants may also produce particulate matter as they burn that may foul or otherwise render inoperable various components of a power system which utilizes a gas turbine. Particulate matter may thus limit the use of solid propellant gas generators in various applications that may require reuse and longevity of a component or system.
As can be seen, there is a need for an improved apparatus and method that may utilize solid propellants for gas generation in emergency power units or systems.